An important feature of electric vehicles is that the vehicle is equipped with a high-voltage power supply that provides energy for the vehicle to run and ensures sufficient dynamic performance, forming a power circuit with high voltage and high current. Compared with the high-voltage system of power plants, substations, electric locomotives and other places or equipment, the power supply system of electric vehicles has its own characteristics. At present, most of the battery packs used in electric vehicles are composed of single cells with a voltage of 3.6V in series and parallel. The working voltage of the high-voltage electrical system of an electric vehicle is usually above 300V, and due to the small impedance of the power transmission line, the high-voltage electrical system The normal operating current may reach tens to hundreds of amperes, and the instantaneous short-circuit discharge current is multiplied. Faults such as short circuit and leakage of high-voltage circuits pose a potential threat to the safety of high-voltage electricity consumption of electric vehicles. When high-voltage circuit insulation failure or short-circuit failure occurs, high voltage and high current may endanger the personal safety of passengers in the car, and also affect the normal operation of low-voltage electrical and vehicle controllers. Therefore, when designing and planning high-voltage electrical systems, the safety of drivers and occupants and the operating environment of vehicles should be fully ensured.
Internal combustion engine vehicles use fuel or natural gas as the power source, and there are few high-voltage insulation problems. As a complex mechatronic product, the electrical system is an important part of electric vehicles, including power batteries, motors, chargers, Components such as energy recovery devices and auxiliary battery charging devices will involve high voltage electrical insulation problems. The working conditions of these components are relatively poor. Vibration, acid-base gas corrosion, temperature and humidity changes may cause rapid aging of power cables and other insulating materials or even insulation damage, which greatly reduces the insulation strength of the equipment and endangers personal safety. . At the same time, since the working voltage of the power battery pack used in electric vehicles is generally above 300V, the higher working voltage puts forward higher requirements for the insulation performance between the high-voltage electrical system of the electric vehicle and the vehicle chassis. If the potential of the vehicle chassis rises due to the decrease in the insulation performance between the two, it will not only endanger the personal safety of the drivers and passengers, but also affect the normal operation of the low-voltage electrical appliances and motor controllers. At the same time, due to the heat accumulation effect of the leakage current loop, the situation In severe cases, the vehicle may even ignite spontaneously.
For a closed-loop high-voltage DC electrical system, its insulation performance is usually characterized by the magnitude of the leakage current from the power supply to the ground in the electrical system. There are two commonly used leakage current detection methods, which are auxiliary power method and current sensing method.
1. Auxiliary power supply method
In the leakage detector used in some electric locomotives, a 110V auxiliary battery is used for detection. The positive electrode of the battery is connected to the negative electrode of the high-voltage DC power supply to be tested, and the negative electrode of the battery is connected to the shell of the locomotive at a single point. When the insulation performance of the system to be tested is good, the battery has no current loop, and the leakage current is zero; when the insulation layer of the power cable is aging or the environment is humid, the battery forms a closed loop through the cable insulation layer, resulting in leakage current. The detector will give an alarm according to the magnitude of the leakage current, and turn off the power of the system to be tested. The disadvantage of this detection method is that it requires a 110V DC auxiliary power supply, which increases the complexity of the system structure, and it is also difficult to distinguish whether the insulation fault is from the positive lead cable or the negative lead cable of the power supply.
Auxiliary power supply block diagram
2. Current sensing method
The leakage detection of the high-voltage DC system can also be performed by using a Hall-type current sensor. The positive and negative poles of the power supply in the system under test are passed through the current sensor together in the same direction. When there is no leakage current, the current flowing from the positive pole of the power supply is equal to the current returning to the negative pole of the power supply. Therefore, the total current passing through the current sensor is zero, and the current The output voltage of the sensor is zero; when leakage occurs, the current sensor has a certain output voltage. According to the positive and negative of the voltage, it can be further judged whether the leakage current generated is from the positive lead cable or the negative lead cable of the power supply. However, the premise of applying this detection method is that the power supply to be tested must be in a working state, and the working current must flow out and in. It cannot evaluate the insulation performance of the power supply to the ground under the no-load state of the power supply.
At present, some electric vehicles use the voltage divider between "DC positive busbar-chassis" and "DC negative busbar-chassis" in the research and development of some electric vehicles to characterize the degree of insulation of the DC busbar relative to the vehicle chassis, but this voltage divider The method can only characterize the relative insulation degree of the DC positive and negative busbars to the chassis, and cannot distinguish the simultaneous reduction of the insulation performance of the DC positive and negative busbars to the chassis. In addition, when there is a large difference in the insulation resistance of the DC positive and negative busbars to the chassis, there may be a misjudgment that the insulation performance is degraded. Therefore, strictly speaking, for the electrical system of an electric vehicle, the electrical safety of the electric vehicle can be guaranteed only by quantitatively detecting the insulation performance of the DC positive and negative busbars to the chassis.